事件和幀通知機制

在前一版本的外部視圖系統 (EVS) 中, IEvsCameraStream接口定義了一個回調方法來傳送捕獲的視頻幀。雖然這簡化了 EVS 服務客戶端實現,但它也使客戶端難以識別任何流式事件,因此難以正確處理它們。為了增強 EVS 開發體驗,AOSP 現在包含一個額外的回調來傳遞流事件。

package android.hardware.automotive.evs@1.1;

import @1.0::IEvsCameraStream;

/**
 * Implemented on client side to receive asynchronous video frame deliveries.
 */
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
    /**
     * Receives calls from the HAL each time a video frame is ready for inspection.
     * Buffer handles received by this method must be returned via calls to
     * IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
     * to IEvsCamera::stopVideoStream(), this callback may continue to happen for
     * some time as the pipeline drains. Each frame must still be returned.
     * When the last frame in the stream has been delivered, STREAM_STOPPED
     * event must be delivered. No further frame deliveries may happen
     * thereafter.
     *
     * @param buffer a buffer descriptor of a delivered image frame.
     */
    oneway deliverFrame_1_1(BufferDesc buffer);

    /**
     * Receives calls from the HAL each time an event happens.
     *
     * @param  event EVS event with possible event information.
     */
    oneway notify(EvsEvent event);
};

此方法提供由三個字段組成的EvsEventDesc

  • 事件的類型。
  • 用於標識事件來源的字符串。
  • 4 個 32 位字數據,包含可能的事件信息。
/**
 * Structure that describes informative events occurred during EVS is streaming
 */
struct EvsEvent {
    /**
     * Type of an informative event
     */
    EvsEventType aType;
    /**
     * Device identifier
     */
    string deviceId;
    /**
     * Possible additional information
     */
    uint32_t[4] payload;
};

並且,為了避免 EVS 和其他 Android 圖形組件之間的圖形緩衝區描述出現任何分歧,已重新定義BufferDesc以使用從 android.hardware.graphics.common@1.2 接口導入的HardwareBufferHardwareBuffer包含HardwareBufferDescription ,它是Android NDK 的AHardwareBuffer_Desc的 HIDL 對應物,具有緩衝區句柄。

/**
 * HIDL counterpart of AHardwareBuffer_Desc.
 *
 * An AHardwareBuffer_Desc object can be converted to and from a
 * HardwareBufferDescription object by memcpy().
 *
 * @sa +ndk libnativewindow#AHardwareBuffer_Desc.
 */
typedef uint32_t[10] HardwareBufferDescription;

/**
 * HIDL counterpart of AHardwareBuffer.
 *
 * AHardwareBuffer_createFromHandle() can be used to convert a HardwareBuffer
 * object to an AHardwareBuffer object.
 *
 * Conversely, AHardwareBuffer_getNativeHandle() can be used to extract a native
 * handle from an AHardwareBuffer object. Paired with AHardwareBuffer_Desc,
 * AHardwareBuffer_getNativeHandle() can be used to convert between
 * HardwareBuffer and AHardwareBuffer.
 *
 * @sa +ndk libnativewindow#AHardwareBuffer".
 */
struct HardwareBuffer {
    HardwareBufferDescription description;
    handle nativeHandle;
}

/**
 * Structure representing an image buffer through our APIs
 *
 * In addition to the handle to the graphics memory, need to retain
 * the properties of the buffer for easy reference and reconstruction of
 * an ANativeWindowBuffer object on the remote side of API calls.
 * (Not least because OpenGL expect an ANativeWindowBuffer* for us as a
 * texture via eglCreateImageKHR().
 */
struct BufferDesc {
    /**
     * HIDL counterpart of AHardwareBuffer_Desc. Please see
     * hardware/interfaces/graphics/common/1.2/types.hal for more details.
     */
    HardwareBuffer buffer;
    /**
     * The size of a pixel in the units of bytes
     */
    uint32_t pixelSize;
    /**
     * Opaque value from driver
     */
    uint32_t bufferId;
    /**
     * Unique identifier of the physical camera device that produces this buffer.
     */
    string deviceId;
    /**
     * Time that this buffer is being filled
     */
    int64_t timestamp;
    /**
     * Frame metadata. This is opaque to EVS manager
     */
    vec<uint8_t> metadata
};

注意: HardwareBufferDescription定義為一個由 10 個 32 位字組成的數組。您可能希望將其轉換為AHardwareBuffer_Desc類型並填寫內容。

EvsEventDescenum EvsEventType的結構,它列出了幾個流事件和一個 32 位字的有效負載,開發人員可以在其中放置可能的附加信息。例如,開發人員可以為流錯誤事件放置錯誤代碼。

/**
 * Types of informative streaming events
 */
enum EvsEventType : uint32_t {
    /**
     * Video stream is started
     */
    STREAM_STARTED = 0,
    /**
     * Video stream is stopped
     */
    STREAM_STOPPED,
    /**
     * Video frame is dropped
     */
    FRAME_DROPPED,
    /**
     * Timeout happens
     */
    TIMEOUT,
    /**
     * Camera parameter is changed; payload contains a changed parameter ID and
     * its value
     */
    PARAMETER_CHANGED,
    /**
     * Master role has become available
     */
    MASTER_RELEASED,
};

幀傳送

通過新的BufferDescIEvsCameraStream還引入了新的回調方法來接收來自服務實現的幀和流事件。

/**
 * Implemented on client side to receive asynchronous streaming event deliveries.
 */
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
   /**
    * Receives calls from the HAL each time video frames are ready for inspection.
    * Buffer handles received by this method must be returned via calls to
    * IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
    * to IEvsCamera::stopVideoStream(), this callback may continue to happen for
    * some time as the pipeline drains. Each frame must still be returned.
    * When the last frame in the stream has been delivered, STREAM_STOPPED
    * event must be delivered. No further frame deliveries may happen
    * thereafter.
    *
    * A camera device will deliver the same number of frames as number of
    * backing physical camera devices; it means, a physical camera device
    * sends always a single frame and a logical camera device sends multiple
    * frames as many as the number of backing physical camera devices.
    *
    * @param buffer Buffer descriptors of delivered image frames.
    */
   oneway deliverFrame_1_1(vec<BufferDesc> buffer);

   /**
    * Receives calls from the HAL each time an event happens.
    *
    * @param  event EVS event with possible event information.
    */
   oneway notify(EvsEventDesc event);
};

較新版本的幀回調方法旨在傳遞多個緩衝區描述符。因此,如果 EVS 攝像頭實現管理多個源,則可以通過單個調用轉發多個幀。

此外,之前通知流結束的協議(發送空幀)已被棄用,並被STREAM_STOPPED事件取代。

事件通知時序圖

圖 1.事件通知序列圖

使用事件和幀通知機制

識別客戶端實現的 IEvsCameraStream 版本

服務可以通過嘗試向下轉換來識別客戶端實現的傳入 IEvsCameraStream 接口的版本:

using IEvsCameraStream_1_0 =
    ::android::hardware::automotive::evs::V1_0::IEvsCameraStream;
using IEvsCameraStream_1_1 =
    ::android::hardware::automotive::evs::V1_1::IEvsCameraStream;

Return<EvsResult> EvsV4lCamera::startVideoStream(
    const sp<IEvsCameraStream_1_0>& stream)  {

    IEvsCameraStream_1_0 aStream = stream;
    // Try to downcast. This will succeed if the client implements
    // IEvsCameraStream v1.1.
    IEvsCameraStream_1_1 aStream_1_1 =
        IEvsCameraStream_1_1::castFrom(aStream).withDefault(nullptr);
    if (aStream_1_1 == nullptr) {
        ALOGI("Start a stream for v1.0 client.");
    } else {
        ALOGI("Start a stream for v1.1 client.");
    }

    // Start a video stream
    ...
}

通知()回調

EvsEvent將通過notify()回調傳遞,然後客戶端可以根據鑑別器識別其類型,如下所示:

Return<void> StreamHandler::notify(const EvsEvent& event) {
    ALOGD("Received an event id: %u", event.aType);
    // Handle each received event.
    switch(event.aType) {
        case EvsEventType::ERROR:
            // Do something to handle an error
            ...
            break;
        [More cases]
    }
    return Void();
}

使用 BufferDesc

AHardwareBuffer_Desc是 Android NDK 的數據類型,用於表示可以綁定到 EGL/OpenGL 和 Vulkan 原語的本機硬件緩衝區。它包含來自先前 EVS BufferDesc 的大部分緩衝區元數據,因此在新的 BufferDesc 定義中替換它。但是,由於這在 HIDL 接口中定義為數組,因此無法直接索引成員變量。相反,您可以將數組轉換為AHardwareBuffer_Desc類型,如下所示:

BufferDesc bufDesc = {};
AHardwareBuffer_Desc* pDesc =
    reinterpret_cast<AHardwareBuffer_Desc *>(&bufDesc.buffer.description);
pDesc->width  = mVideo.getWidth();
pDesc->height = mVideo.getHeight();
pDesc->layers = 1;
pDesc->format = mFormat;
pDesc->usage  = mUsage;
pDesc->stride = mStride;
bufDesc_1_1.buffer.nativeHandle = mBuffers[idx].handle;
bufDesc_1_1.bufferId = idx;